The present invention relates to a film-forming apparatus and a method for forming a film and more particularly to a film-forming apparatus suitable for carrying out the CVD technique such as the metal organic chemical vapor deposition (MOCVD) technique as well as a method for forming a metal oxide film using the foregoing apparatus.
When forming a thin film according to the CVD technique such as the MOCVD technique, it is in general necessary to remove, from raw gases (material gases), fine particles including, for instance, foreign fine particulate substances and water to an amount as low as possible in order to satisfy such recent requirements that the semiconductor element should have a high degree of integration. Up to this time, it has, for instance, been proposed to introduce raw gases into a film-forming chamber (a reaction chamber) as a vacuum chamber after passing them through a filter member having high filtering characteristics for the purpose of the removal of these particles to a ppt level.
In general, when forming a thin film in accordance with the MOCVD technique, a mixed gas (hereunder also referred to as film-forming gas) consisting of vaporized raw gas and a reactive gas is introduced into a film-forming chamber as a vacuum chamber and a thin film is formed on the surface of a substrate placed on a stage disposed within the vacuum chamber through a vapor phase chemical reaction. On the top face of the film-forming chamber, a gas head such as a shower plate is set up in such a manner that it is opposed to the stage and a gas-mixing chamber is fitted to this gas head. The raw gas and reactive gas fed to the mixing chamber when forming a film are introduced into the film-forming chamber, which has been evacuated to a predetermined degree of vacuum, through the gas head and then a thin film is formed on the substrate surface through a chemical reaction.
In this case, it is necessary to establish the uniform distribution of the flow rate and the concentration of the film-forming gas as well as the uniform temperature distribution in the film-forming chamber and the region just above a substrate and the amount of fine particles present in the film-forming gas, in particular, the raw gas should be reduced to a level as low as possible in order to form a thin film having uniform film quality and uniform film thickness distribution on the substrate surface.
Even if the raw gas is introduced into the reaction chamber after passing it through a filter member upon the formation of a thin film according to the MOCVD technique as has been discussed above, however, problems arise. For instance, such particles are not completely removed by simply passing the raw gas through a filter member and the service life of the filter member used in the foregoing treatment is quite short.